Agricultural productivity is severely affected by soil salinity. One possible mechanism by which plants could survive salt stress is to compartmentalize sodium ions away from the cytosol. Overexpression of a vacuolar Na+/H+ antiport from Arabidopsis thaliana in Arabidopsis plants promotes sustained growth and development in soil watered with up to 200 millimolar sodium chloride. This salinity tolerance was correlated with higher-than-normal levels of AtNHX1 transcripts, protein, and vacuolar Na+/H+ (sodium/proton) antiport activity. These results demonstrate the feasibility of engineering salt tolerance in plants.

Ca2+ ions play a vital role as second messengers in plant cells during various developmental processes and in response to environmental stimuli. Plants have evolved a diversity of unique proteins that bind Ca2+ using the evolutionarily conserved EF-hand motif. The currently held hypothesis is that these proteins function as Ca2+ sensors by undergoing conformational changes in response to Ca2+-binding that facilitate their regulation of target proteins and thereby co-ordinate various signalling pathways. The three main classes of these EF-hand Ca2+sensors in plants are CaMs [calmodulins; including CMLs (CaM-like proteins)], CDPKs (calcium-dependent protein kinases) and CBLs (calcineurin B-like proteins). In the plant species examined to date, each of these classes is represented by a large family of proteins, most of which have not been characterized biochemically and whose physiological roles remain unclear. In the present review, we discuss recent advances in research on CaMs and CMLs, CDPKs and CBLs, and we attempt to integrate the current knowledge on the different sensor classes into common physiological themes.

Calmodulin CaM is the most prominent Ca2+ transducer in eukaryotic cells, regulating the activity of numerous proteins with diverse cellular functions. Many features of CaM and its downstream targets are similar in plants and other eukaryotes. However, plants possess a unique set of CaM-related proteins, and several unique CaM target proteins. This review discusses recent progress in identifying plant-specific CaM-binding proteins and their roles in response to biotic and abiotic stresses and development. The review also addresses aspects emerging from recent structural studies of CaM interactions with target proteins relevant to plants.

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SummaryThe complexity of Ca 2 + patterns observed in eukaryotic cells, including plants, has led to the hypothesis that specific patterns of Ca 2 + propagation, termed Ca 2 + signatures, encode information and relay it to downstream elements (effectors) for translation into appropriate cellular responses. Ca 2 + -binding proteins (sensors) play a key role in decoding Ca 2 + signatures and transducing signals by activating specific targets and pathways. Calmodulin is a Ca 2 + sensor known to modulate the activity of many mammalian proteins, whose targets in plants are now being actively characterized. Plants possess an interesting and rapidly growing list of calmodulin targets with a variety of cellular roles. Nevertheless, many targets appear to be unique to plants and remain uncharacterized, calling for a concerted effort to elucidate their functions. Moreover, the extended family of calmodulin-related proteins in plants consists of evolutionarily divergent members, mostly of unknown function, although some have recently been implicated in stress responses. It is hoped that advances in functional genomics, and the research tools it generates, will help to explain the multiplicity of calmodulin genes in plants, and to identify their downstream effectors. This review summarizes current knowledge of the Ca 2 + -calmodulin messenger system in plants and presents suggestions for future areas of research. AbbreviationsCaM, clamodulin; Ca 2 + /CaM, Ca 2 + -bound CaM; CaMBD, CaM-binding domain; CCaMK, chimeric Ca 2 + /CaM-dependent protein kinase; CDPK, calcium-depentent (calmodulin-independent) protein kinase; CNGC, cyclic nucleotide gated channel; cNMP, cyclic nucleotide monophosphate; KCBP, kinesin-like CaM-binding protein.© New Phytologist (2001) 151 : 35 -66